Photoelectrical control system for color television receivers



Oct. 27, 1959 5, w, MQULTQN T 2,910,615

PHOTOELECTRIC CONTROL SYSTEM FOR COLOR TELEVISION RECEIVERS Filed May31, 1955 2 Sheets-Sheet 2 Low PASS 42 VARIABLE nun PHASE nc) SHIFTERHORQIVERT.

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LV w :/"L16 v B+ 4 g F74 2. mm L) RECEIVER +L 36 g I f $rPfiJ A r0- r l:MELVIN 5. fi/mr/N nun PHOTOELECTRICAL CONTROL SYSTEM FOR COLORTELEVISION RECEIVERS Stephen W. Moulton, Hatboro, and Melvin E. Partin,Philadelphia, Pa., assignors to Philco Corporation, Philadelphia, Pa., acorporation of Pennsylvania Application May 31, 1955, Serial No. 511,858

6 Claims. (Cl. 315-10) This invention relates to an improved colortelevision system and, more particularly, to a color television receiversystem of the type which utilizes the screen structure of a singlecathode ray tube for the formation of the colored image.

Screen structures suitable for the aforenoted purpose are formed of alarge number of minute phosphor elements, different ones of which areresponsive to electron impingement to emit light in different primarycolors, e.g. red, green and blue. These phosphor elements are arrayed insuch manner that the electron beam, during its normal scanning traversalof the screen structure, impinges upon elements emissive of light ofthese different colors in rapidly recurrent sequence. It is apparcutthat, in a receiver system utilizing such a screen structure, theelectron beam intensity must be controlled by means of a signal whichrepresents intelligence concerning a particular color during the sameintervals during which the electron beam impinges upon phosphor elementsemissive of light of that same color. Furthermore, in order to obtainvisual blending of the light emitted from successively impinged phosphorelements, these elements must be impinged in extremely rapid succession.As a result the time interval during which the electron beam dwells uponany particular phosphor element is obviously extremely short. This, inturn, makes it difiicult to maintain the desired synchronism between thetime intervals during which beam intensity is controlled by a signalrepresentative of a given color and the time intervals during which thebeam impinges upon screen elements emissive of light of that color. Toovercome this difliculty it has been proposed to derive, from the screenstructure, indications of electron beam impingement upon portions of thescreen structure bearing predetermined geometrical rela tionship to thecolored light emissive phosphor elements. This has been done byphotoelectric or by secondary electron emissive means disposed so as tosense such impingement and to produce electrical signals (calledindexing signals) indicative thereof. The indexing signals thus producedwere then utilized to control the rate of application of colorrepresentative signals to the electron beam in such manner as to providethe desired synchronism.

It is apparent that, in the absence of further precautions, theaforementioned indexing signals vary not only as a function of beamposition but also as a function of beam intensity (as determined by thepicture content of the televised scene). Since the presence of thesevariations due to picture intelligence reduces the utility of theindexing signals in producing the desired synchronism, elaborateprecautions were taken to eliminate tube, all for the purpose ofrendering the indexing sig- 2,910,615 Patented Oct. 27,1959

nals more readily distinguished from contaminating signals. Suchprecautionary measures complicated the receiver system and rendered thesame more costly as Well as more diflicult to adjust and to maintain inadjustment over prolonged periods of operation.

Accordingly it is a primary object of the invention to provide animproved and simplified color television receiver.

It is another object of the invention to provide an improved andsimplified color television receiver characterized by the use of asingle cathode ray tube screen structure to reproduce a televised imagein full color.

It is still another object of the invention to provide a colortelevision receiver characterized by the use of a single cathode raytube screen structure for the formation of the televised image in fullcolor and further characterized by the utilization, in a new andimproved manner, of electrical signals derived from this screenstructure in response to electron beam impingement thereon.

To achieve the foregoing objects, as well as others which will appear,we provide a color television receiver which includes a screen structureof the multi-color phosphor element type hereinbefore briefly describedwith apparatus which is responsive to electron beam impingement uponphosphor elements emissive of light in the different colors to producedifferent electrical signals, respectively representative of the timesduring which the beam impinges on these different elements and also ofthe intensities with which such impingements take place. We provideadditional means for comparing each of these different beam impingementrepresentative signals with a signal derived from the received colortelevision signal and representative of received picture informationconcerning the same color as the particular beam impingementrepresentative signal involved in the comparison. We also provide meanswhich are responsive to the existence of a discrepancy between theaforedescribed signals under comparison to modify the beam impingementrepresentative signal so as to reduce this discrepancy (and preferablyso as to eliminate the same entirely). From the separate modifiedsignals thus produced we derive a combined signal corresponding to thealgebraic sum of the individual modified signals and we supply thiscombined signal to the cathode ray tube as its beam intensity controlsignal.

The details of construction and operation of specific apparatusembodying the aforedescribed principles will be better understood fromthe following discussion taken -in conjunction with the accompanyingdrawings wherein:

Figure 1 illustrates those portions of a color television receiver whichembody one form of my invention;

Figure '2 shows an improved form of certain portions of the system ofFigure l; and V Figure 3 showsa preferred embodiment of my invention ina color television receiver.

Referring now to the embodiment of Figure 1 the color televisionreceiver system illustrated therein comprises a receiver portion 10which is supplied with signals intercepted by an antenna 11. In thediscussion which follows, these intercepted signals are assumed to be ofthe form which is now standard for this country. However it will beunderstood that our invention is broadly applicable to any form of colortelevision signal, since any such signal must necessarily contain thesame sort of information as the standard signal. This receiverportion 10may be of any conventional form suitable for converting the receivedsignal into three separate output signals, which appear respectively inoutput leads 12, 13 and 14 and which represent respectively thedifferent primary color components of the televised scene (eg the red,

green and blue components). Thus receiver portion may comprise a radiofrequency amplifier, a heterodyne detector for converting the receivedradio frequency signal into an intermediate frequency signal, anappropriate number of intermediate frequency amplifier stages and asecond detector, all of which may be of any conventional form. Inaddition receiver portion 10 may include conventional synchronousdemodulators, matrixing circuits this connection.

Another component of the receiver system of Fig. l which may be ofconventional construction is the cathode ray tube 15. More particularlythis cathode ray tube may comprise a conventional cathode 16, a beamintensity control grid 17, afirst anode 18 which is supplied with asuitable positive potential from a conventional source A+ of suchpotential, a second anode 19 which may be in the form of a conductivecoating on the interior of the funnel shaped portion of tube and whichis supplied with a suitable positive potential from a conventionalsource A++ of such a potential, and a screen structure 20. This screenstructure 20 may be composed of a large number of parallel phosphorstrips disposed with their longitudinal axes transverse to thehorizontal line scanning direction of the electron beam projected fromcathode 16. Different ones of these strips are made of phosphormaterials responsive to electron impingement to emit light in the samethree primary colors (red, green and blue in the present example) as arerepresented by the signals appearing respectively in output leads 12, 13and 14 of receiver portion 10. These phosphor strips are disposed inrecurrent sequence across the screen structure, so that the electronbeam impinges upon phosphors emissive of light of the different colorsin rapidly recurrent sequence during its horizontal line scans acrossthe screen structure. The aforedescribed phosphor strips arediagrammatically illustrated in Fig. 1 by vertical lines 21. It will beunderstood that, in practice, a much larger number of phosphor stripsforms the screen structure of the cathode ray tube than is representedby the vertical lines 21. In fact the number of phosphor strips ispreferably coordinated with the rate of horizontal beam deflectionacross the screen structure in such manner that the electron beamtraverses successive strips emissive of light of any particular primarycolor at a rate of approximately 7 megacycles. In the case of a screenstructure approximately 16 inches wide by 12 inches high, this requiresthe provision of a screen structure having approximately 400 phosphorstrips of each color.

The cathode ray tube 15 is also equipped with conventional deflectioncoils 22 which are, in turn, supplied with the usual horizontal andvertical deflection signals from a source 23 of such signals. The sourceof deflection signals receives synchronizing signals in the usual mannerfrom receiver 10.

In accordance with our invention there are disposed, in lightcommunicating relationship with the screen structure 20, threephotosensitive devices, designated 24, and 26, respectively, each ofwhich is of such construction that it responds only to light of one ofthe three different primary colors emitted by the phosphor strips of thescreen structure. More particularly photosensitive device 24 may beconstructed to be responsive substantially exclusively to blue lightemitted from the screen structure, while device 25 may be maderesponsive to green light and device 26 to red light. Thesephotosensitive devices may be of any desired conventional form. Forexample they may consist of conventional phototransmissive of light onlywithin the desired portions of the color spectrum. The output circuitsof these photosensitive devices are respectively connected to three highpass filters 27a, 27b and 27c and also to three detectors 28a, 28b and28c. The output circuits of the high pass filters are connected tolimiters 29a, 29b and 290 respectively and the output circuits of theselimiters are re spectively connected to one input circuit of each ofmodulators 30a, 36b and 30c. The output circuits of the detectors, onthe other hand, are respectively connected to one input circuit of eachof subtractors 31a, 31b and 310 and the output circuits of thesubtractors are in turn connected respectively to the other inputcircuit of each of modulators 30a, 30b and 300. The output circuits ofall three modulators are connected to the beam intensity control gridelectrode 17 of cathode ray tube 15. The output leads 12, 13 and 14 ofreceiver portion 10 which, as will be recalled, bear the red, green andblue color representative signals, respectively, are connected to thesecond input circuit of subtractors 31c, 31b and 31a, respectively. Eachof the aforementioned detectors, subtractors, high pass filters,limiters and modulators may be of any conventional form and needtherefore not be described further.

The aforedescribed system operates as follows. Consider, for example,the blue light responsive photosensitive device 24. This device receivesa flash of blue light from the screen structure 20 each time that theelectron beam, during its scanning of the screen structure, traverses aphosphor strip emissive of blue light. The intensity of this flash ofblue light will, of course, depend upon the intensity of the blue lightemitted by the phosphor strip and therefore also upon the blue lightcontent of the particular picture element which is being reproduced atthat instant. Because of the aforementioned coordination between thenumber of phosphor strips in the screen structure and the rate ofscanning of this screen structure, successive ones of these flashes ofblue light will occur at a rate of approximately 7 megacycles.Accordingly there will be produced in the output circuit ofphotoelectric device 24 :a signal component of approximately 7 megacyclefrequency subject to amplitude variations which reflect intensityvariations in the blue light flashes from screen structure 20. The timephase position of this signal component will correspond to the timephase position of successive traversals of the blue light emissivephosphor strips by the electron beam. Passage of this signal fromphotosensitive device 24 through high pass filter 27a (which isconstructed in conventional manner to transmit only signals in excess ofapproximately 6.5 megacycles) and through limiter 29a (which isconventionally constructed to suppress all amplitude variations in thesignal supplied thereto from high pass filter 27a) will produce, at theinput of modulator 30a, a 7 megacycle signal having the time phaseposition determined by the output signal of photoelectric device 24 andhaving a fixed amplitude. On the other hand, application of the sameoutput signal from photoelectric device 24 to detector 28a (which isconventionally constructed to produce a unidirectional output signalhaving variations indicative of the variations in the amplitude of the 7megacycle signal from the photoelectric device) and subtractivecombination in subtractor 31a of this detected signal with the bluelight representative signal supplied from receiver 10 by way of lead 14,will pro duce at the output of the subtractor a signal which representsthe extent to which the intensity of blue light emitted by the screenfrom a particular picture element departs from the intensity with whichit is desired to have this element of the screen structure emit (thedesired value being, of course, represented by the amplitude of thereceived, blue light representative signal). This error signal fromsubtractor 31a is then utilized, in modulator 30a, to modulate theamplitude of the fixed amplitude'7 multiplier tubes, respectivelyequipped with optical filters megacycle signal supplied to thismodulator from limiter 29a, thereby producing, at the output ofmodulator 30a,

I a signal having a component at approximately 7 meg-acycles whose timephase position is determined by time phase position of successive actualtraversals of the blue light emissive phosphor strips by the electronbeam of the cathode ray tube and whose amplitude is precisely such thatsubtractor 31a will produce zero output, thus designating the productionof blue light in the screen structure with precisely the intensityindicated by the received signal.

The operation of the circuits connected to green responsivephotosensitive device 25 and to red responsive photosensitive device 26,respectively,is similar to that described for the circuit connected tothe blue light responsive photosensitive device 24. Accordingly thethree modulators 30a, 30b and 300 will produce, respectively, the properoutput signals for driving the beam intensity control grid electrode 17in such manner as to cause the emission of red, green and blue light ofproper intensity from the screen structure. As a result there will bereproduced, on screen structure 20, an image in full color which is anaccurate replica of the televised scene, as represented by the red,green and blue light representative output signals from receiver 10.

Whenever certain simple forms of photoelectric devices, such as, forexample, conventional two-electric photoelectric tubes are used for thephotoelectric devices 24, 25 and 26 in a receiver system embodying ourinvention, the comparatively complicated signal utilization circuitsillustrated in Fig. l are necessary. However we have found thatconsiderable circuit simplification may be obtained by using a moreadvanced form of photoelectric device to observe the cathode ray tubescreen structure. The manner in which these simplifications may beCarried out is illustrated in Fig. 2 of the drawings to which referencemay now be bad. There is shown in Fig. 2 only that portion of the entirereceiver system which differs from the system of Fig. 1, together withthe necessary indications of how these portions are connected with thoseportions of the system of Fig. 1 which have not been illustrated in Fig.2. More particularly there are shown in Figure 2 three photo-multipliertubes 35, 36 and 37, each of which is of conventional construction andtherefore comprises the usual photoemissive cathode, a multiplicity ofdynodes, and an anode. These photo-multiplier tubes are disposed inthose positions with respect to the screen structure of the receivercathode ray tube which are occupied by the photoelectric devices 24, 25and 26 in Figure 1tl1at is, they are positioned so as to be illuminatedby light emanating from this screen structure as the electron beam scansits conventional raster upon the screen. In addition, thephotomultiplier tubes of Fig. 2 are also constructed so that they arerespectively responsive substantially exclusively to diflerent ones ofthe three primary colors of light emitted by the screen structure. Thisphoto-multiplier tube 35 may be constructed to respond substantiallyexclusively to blue light, while photo-multiplier tube 36 responds togreen light and photo-multiplier 37 responds to red light.

To this end the photoemissive cathodes of the respectivephoto-multiplier tubes may be constructed of suitably color responsivesubstances or, alternatively, appropriately color-selective filters maybe interposed in the path of light falling from the screen structureupon these photo-cathodes. The anodes of all of the photomultipliertubes are connected to a low-pass filter 38 which may be conventionallyconstructed to transmit only .signal variations emanating from theindividual photomultiplier tubes at frequencies up to about 7megacycles.

'The output of the filter 37 is, in turn, connected to the 'beamintensity control grid electrode 17 of the cathode ,a suitable loadresistor, to a source of conventional positive potential B+, anddifferent ones of these last dynodes are connected to different ones ofthe output leads 12', 13 and 14 of the receiver 10 of Fig. 1. Moreparticularly the last dynode of photo-multiplier tube 35 is connected tolead 14 which, it will be recalled, bears the blue representativeportion ofthe received intelligence signal. The last dynode ofphoto-multiplier 36, on the other hand, is connected to output lead 13and is therefore supplied with green representative signals and the lastdynode of photo-multiplier tube 37 is supplied with red representativesi nals by way of lead 12. It may be shown that, by appropriateadjustment of the values of the resistors connecting these dynodes totheir respective sources of positive potential B+ and by appropriateselection of the values of these positive potentials, the aforedescribedcircuitry will effect a modulation of the 7 megacycle signal flowingfrom each photocathode to the anode of the correspondingphoto-multiplier tube which is substantially analogous to the modulationof the 7 megacycle signal in modulators 30a, 301) or 30c of Fig. 1. Moreparticularly, at the last dynode of each photo-multiplier tube therewill be formed a difference voltage between the average value of the 7megacycle signal developed by the corresponding photoemissive cathodeand the corresponding color representative signal from the receiver, andthis difference voltage will modulate the entire photo-multiplier anodecurrent and hence the 7 megacycle component thereof.

In each of the receiver systems of Figures 1 and 2 it is necessary toderive, from the received signal, separate signals respectivelyrepresentative of the different primary colors of the televised scene.While, as has been indicated, this can readily be accomplished by meansof entirely conventional apparatus it is sometimes preferable to utilizethe received signal in a form which requires less modification. Moreparticularly it is well known that the standard color television signalwhich is now standard for this country is composed of a componentoccupying principally the 0 to 3 megacycle frequency range, whichisrepresentative of the luminance of the televised scene, and a componentin the form of a subcarrier wave of approximately 3.58 megacycle nominalfrequency which is subject to amplitude and phase modulationrepresentative of the chrominance of the televised scene. This lattercomponent has sidebands extending approximately 0.6 megacycles on eitherside of the nominal 3.58 megacycle carrier frequency. In addition thereceived signal includes the usual horizontal and vertical scanningsynchronizing pulses, as well as so-called color synchronizing burstseach of which consists of a few cycles of a carrier wave having the samefrequency as the unmodulated chrominance snbcarrier and having referencephase and amplitude therefor. Each such color burst is superposed on thetrailing portion or backporch of a horizontal line blanking pulse, theleading portion of which is occupied by the conventional horizontal linesynchronizing pulse. All of the foregoing is well known and isrecapitulated here only for completeness of description.

A receiver system embodying our invention in which a signal of theaforedescribed form is utilized without decomposition of the same intoseparate signals, respec tively representative of the different primarycolors of the televised scene, is illustrated in Figure 3 of thedrawings to which reference may now be had.

The system there illustrated comprises a receiver por tion 40 whichincludes those portions of the receiver system up to and including thesecond detector. Thus the receiver portion 40 may include the usualradio frequency amplifiers, converters, intermediate frequencyamplifiers and second detector, all of which may take any one of avariety of conventional forms. As a result of the conventional operationof these elements, a standard signal intercepted by antenna 41 andsupplied to receiver 40 appears at the output of the latter reduced toits 'stantial exclusion of all other signals.

' components and is therefore made transmissive of signals in the 3 to4.2 megacycle frequency range to the sub- Finally color burst separator44 is constructed to separate the aforementioned color synchronizingbursts from the remainder of the signals. A variety of circuits suitablefor this latter purpose are known. For example, color burst separator 44may include a triode biased sufliciently negative so that only theapplication of the horizontal line blanking pulses drive the same intoconduction, and a filter transmissive only of signals of 3.58 megacyclefrequency to the substantial exclusion of signals at all otherfrequencies. The intermittent signal of 3.58 megacycles frequency and ofreference amplitude and phase for the modulated chrominance subcarrier,which is produced by color burst separator 44, is utilized tosynchronize, in frequency and phase, a cohered oscillator 45 which maybe of any conventional construction and which will produce a continuousoutput signal also of reference frequency and phase for the chrominancesub-carrier. The output signal from this cohered oscillator 45 issupplied to a mixer 46 in which it is hererodyned with the output signalfrom an oscillator 47, which may be of any conventional constructionsuitable for the production of a signal at a frequency substantially inexcess of any video frequency encountered in the receiver system, e.g.at 100 megacycles. The sum frequency heterodyne component produced bythe operation of mixer 46 is selectively derived from this mixer and issupplied to a second mixer 48 in which it is heterodyned with thechrominance signal derived from bandpass filter The resultant differencefrequency heterodyne components are then supplied, by way of a variablephase shifter 49, to one dynode 50 of a photo-multiplier tube 51 havingthere separate photoemissive cathodes 52, 53 and 54 so disposed withinthe photo-multiplier tube that their emission currents flow through acommon set of dynodes including dynode 50 to a common anode 55. Thisphoto-multiplier tube 51 is disposed in such spatial relationship to acathode ray tube 56 (which may be exactly identical in every respectwith the cathode ray tube of the systems of Figs. 1 and 2) as to beimpinged by colored light emitted from screen structure 57 thereof. Inaddition provisions are made so that the photoemissive cathode 52 of thephoto-multiplier tube 51 is responsive only to blue light emanating fromthe screen structure of the cathode ray tube while the photoemissivecathode 53 is responsive substantially exclusively to green light andphotoemissive cathode 54 is responsive only to red light. This may againbe accomplised either by appropriate construction of the respectivephotoemissive cathodes or by the interposition of optical filters in thepath of the light falling thereupon. Assuming an equal spacing betweenthe different phosphor strips constituting the screen structure 57 ofcathode ray tube 56, the photoemissive cathodes are further suppliedwith signals from oscillator 47 diifering in phase from each other by120 degrees. More particularly, if a blue light emissive phosphor stripis disposed so as to be flanked by a red light emissive phosphor stripin the direction of approach of the scanning electron beam and by agreen light emissive phosphor strip in the direction of the recedingelectron beam, then the blue light responsive photo-cathode 52 may besupplied with the signal from oscillator 47 directly, without phaseshift while the signal supplied to photo-cathode 53 delayed by 120degrees in phase shifter 58 and the signal supplied to-photoemissivecathode 54 .is advanced by 120 degrees in phase shifter 59, both ofwhich may be of entirely conventional construction. The anode 55, whichreceives the emission currents from all three photo-emissive cathodes(as has been indicated), is connected by way of a variable phase shifter60 to the beam intensity control grid electrode 61 of cathode ray tube56, to which is also connected the output circuit of low pass filter 42.

The operation of the aforedescn'bed system is as follows. At the outputof low pass filter 42 there is produced the received luminance signalwhich is then applied to beam intensity control grid electrode 61. Atthe output of mixer 48 there is produced the received chrominance signalsuperposed, not upon the 3.58 megacycle subcarrier upon which it isreceived and transmitted by bandpass filter 43, but rather upon amegacycle subcarrier derived from oscillator 47 by the doubleheterodyning action of mixers 46 and 48. This chrominance signal at 100megacycles nominal frequency is further heterodyned with certain sum anddifference frequency heterodyne components present in the stream ofelectrons flowing through the dynodes of photo-multiplier tubes by theapplication of the chrominance signal to dynode 50 in the mannerpreviously described. These heterodyne components of photo-multipliercurrent are generated by the interaction, at the three photoemissivecathodes of the tube, between the 7 megacycle variation in thephotoemitted current in response to scanning of the screen structure bythe electron beam of cathode ray tube 56 and the 100 megacycle signalssupplied to these photoemissive cathodes fro-m oscillator 47, also in amanner hereinbefore described. The aforedescribed interaction betweenthe 100 megacycle signals and the 7 megacycle signals at thephotoemissive cathodes will produce sum and frequency heterodynecomponents at nominal frequencies of 93 and 107 megacycles,respectively, each modulated in phase and amplitude with time phaseinformation concerning the rate of scan of the electron beam across thedifferent phosphor strips of the screen structure 57 and withinformation concerning the intensity of the light emitted from thesedifferent phosphor strips. The aforenoted heterodyne interaction betweenthese sum and frequency heterodyne components and the 100 megacyclesubcarrier wave modulated with chrominance intelligence from mixer 48will result in the production, at the common anode 55, of a 7 megacyclecomponent constituted of (1) the sum frequency heterodyne componentbetween the 93 megacycle component and the 100 megacycle chromamodulated component and (2) the difierence frequency heterodynecomponent formed by interaction between the 107 megacycle component andthe 100 megacycle chrominance modulated signal. In addition there willappear at the common anode of photomultiplier tube 51 a 7 megacyclecomponent equal to the sum of the original 7 megacycle components ofelectron stream intensity variations produced by the scanning of thescreen structure by the electron beam of cathode ray tube 56. It may beshown that, by appropriate phasing adjustment of variable phase shifter49 and variable phase shifter 60, there will be produced, as a result ofall of the aforedescribed interactions, a signal at the output ofvariable phase shifter 60 which is in the form of a carrier wave ofnominal frequency equal to the rate at which the electron beam of thecathode ray tube 56 traverses phosphor strips emissive of light of aparticular color, and which bears such amplitude and phase modulation aswill cause the emission, from the screen structure, of light of allthree primary colors corresponding precisely to that intensity of thesethree primary colors which is indicated by the received color televisionsignal.

In each of the aforedescribed embodiments the apparatus for derivingindications of the intensity with which ill? filfictron beam of thecathode ray tube impinges upon portions of its screen structure whichare emissive of light of different colors has included devices which aresensitive to that light itself. It will be understood, however, thatother devices may be used for this purpose. More particularly it isentirely feasible to incorporate, in the screen structure of the cathoderay tube, strip-like elements disposed in geometrical coincidence withthe phosphors emissive of light of difierent colors respectively andresponsive to electron impingement to emit different invisibleradiations such as, for example, different Wave lengths within theultra-violet spectrum. The radiation from the screen structure wouldthen be sensed by appropriately sensitized devices which would operateto produce signals similar to those produced by the photoelectricdevices illustrated and which would be utilized in a similar manner. Infact the aforementioned ultraviolet light emissive substances may beincorporated directly in the phosphor strips of the screen structure byiixing them with the phosphors prior to their deposition, in which casegeometrical registry between the ultraviolet light emissive substancesand the proper phosphor strips will be inherently assured.

The use of such separate materials for energizing the beam perceptivedevices may prove particularly desirable because the phosphorsthemselves preferably have long radiation persistence whereas shortradiation persistence is desirable to excite the maximum 7 megacyclecomponent within the beam perceptive devices. Accordingly it may beadvantageous to use ultra-violet light emissive substances having muchshorter radiation persistence than the phosphor materials themselves.

It will be apparent that still other modifications of the particularapparatus illustrated will occur to those skilled in the art withoutdeparting from our inventive concept and accordingly we desire thelatter to be limited in scope only by the appended claims.

We claim:

1. In a color television receiver which includes a cathode ray tubeadapted for the emission of different radiations in response toimpingement of the electron beam sequentially upon different portions ofits screen structure, said radiations being emitted with intensitiesdetermined by the intensity of said beam: means for producing at leastone electrical signal indicative of the intensities of said radiationsand of the intervals during which they occur; means for comparing saidproduced signal with a signal indicative of the desired intensities ofsaid radiations during corresponding time intervals; means responsive tothe existence of a discrepancy between said compared signals to modifysaid produced signal; and means for utilizing said modified signal tocontrol the intensity of said beam.

2. In a color television receiver which includes a cathode ray tubeadapted for the emission of light of different colors in response toimpingement of the electron beam sequentially upon dilferent portions ofits screen structure, said light being emitted with intensitiesdetermined by the intensity of said beam: means responsive to emissionsof light of at least one of said colors to produce at least oneelectrical signal indicative of the intensities of said emissions and ofthe intervals during which they occur; means for comparing said signalwith a signal indicative of the desired intensities of said emissionsduring corresponding intervals; means responsive to the existence of adiscrepancy between said compared signals to modify said producedsignal; and means for utilizing said modified produced signal to controlthe intensity of said beam.

3. In a color television receiver which includes a cath ode ray tubeadapted for the emission of different radiations in response toimpingement of the electron beam sequentially upon different portions ofits screen structure, said radiations being emitted with intensitiesdetermined by the intensity of said beam: means for producing at leastone electrical signal indicative of the intensities of said emittedradiations and of the intervals during which they occur; means forcomparing said produced signal with a signal indicative of the desiredintensities of said radiations during corresponding intervals; meansresponsive to the existence of a discrepancy between said comparedsignals to modify said produced signal and to reduce said discrepancy;and means for utilizing said modified signal to control the intensity ofsaid beam.

4. In combination: a cathode ray tube having a source of an electronbeam and a screen structure with different portions respectivelyresponsive to electron beam impingement to emit different radiations,each with intensity determined by the intensity of the impingingelectron beam; means for deflecting said beam so as to cause it toimpinge upon said different portions during different intervals; meansresponsive to said radiations to produce at least one electrical signalindicative of the intensities of said radiations and of the intervalsduring which they are emitted; means for comparing said signal with asignal indicative of the desired intensities of said radiations duringcorresponding intervals and for producing an error signal indicative ofdiscrepancies between said signals; means responsive to said errorsignal to modify said radiation produced signal; and means for utilizingsaid modified signal to control the intensity of said beam.

5. The combination of claim 1 characterized in that said meansresponsive to said radiations comprises photosensitive devices disposedin radiation communicating relationship to said screen structure andresponsive to the emissions of said radiations to produce saidelectrical signal indicative of the intensity of radiation.

6. In combination: a cathode ray tube having a source of an electronbeam and a screen structure with different portions respectivelyresponsive to electron beam impingement to emit different radiations,each with intensity determined by the intensity of the impingingelectron beam; means for deflecting said beam so as to cause it toimpinge upon portions emissive of the same radiation at a predeterminedrate and to impinge upon portions emissive of different radiationsduring intervals occupying predetermined relative time-phase positions;an electron multiplier tube disposed in radiation communicating relationto said screen structure, said tube having a plurality of cathodesrespectively responsive to different ones of said radiations to emitelectrons in numbers determined by the intensities of said radiations, acommon anode for intercepting electrons flowing in said tube, and acommon set of dynodes between said cathode and said anode; means forapplying to said cathodes, respectively, alternating signals of the samefrequency and in the same relative phases as said impingements ofdifferent screen portions by said electron beam; means for applying toone of said dynodes an alternating signal of said predeterminedfrequency and representative at time intervals during any given cyclewhich are also in said relative time phase positions of the desiredvalues of said diiferent radiations; means for deriving, from saidanode, an alternating signal having a nominal frequency equal to saidrate of beam traversal of successive portions emissive of the sameradiation; and means for applying said derived signal to said cathoderay tube to control the intensity of said electron beam.

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